1. Field of the Invention
The invention relates to a process and to its use for the combined removal of waste gas flows containing ammonia and nitrogen oxides in an industrial plant by selective non-catalytic reduction in a temperature range from 850° C. to 1100° C. or by selective catalytic reduction in a temperature range from 150° C. to 550° C. In this process conversion of ammonia and nitrogen oxides (NOx) by previous mixing of two or more waste gas flows obtained in the industrial plant, the temperature of the mixture and the reduction rates of ammonia and nitrogen determining whether selective non-catalytic reduction or selective catalytic reduction is used. In this novel process, the two noxious substances (ammonia and nitrogen oxides) contained in the process waste gases serve one another as reaction partners for the purpose of an efficient mutual depletion, which is more cost effective than separate purification of the individual waste gas flows.
2. Description of the Related Art
In some branches of industry as, for example, in the fertiliser production, various process sections produce waste gas flows which are polluted with ammonia and/or nitrogen oxides and are to be purified before being discharged into the ambient air because of the environmental and olfactory pollution caused by such compounds. Such waste gas flows are, for instance, waste gas flows from the low-pressure absorber of a urea synthesis plant. Such waste gas flows pose a problem in so far as their emission values are far above the concentrations recommended by the Technical Instructions on Air Quality Control, First General Administrative Regulation pertaining to the Federal Air Pollution Control Act, as of 2002. According to these a value of 50 mg/Nm3, for example, is required for ammonia. Also partly contained in the waste gas are lower alkanes such as methane which is a strong greenhouse gas.
In the processes according to patent literature ammonia is removed from the waste air by addition of sulphuric acid or nitric acid. Such process is also known by the name of “acid scrubbing” as described in U.S. Pat. No. 3,607,022, for example. The ammonia is removed from the waste air flow by chemical absorption and in the presence of the acid solvent converted into the corresponding ammonium salt. If nitric acid is used, ammonium nitrate will be formed, if sulphuric acid is added, ammonium sulphate will be formed. The acid scrubbing process thus involves a very high demand for equipment and logistics, as—apart from the complex scrubber—the necessary quantities of acid have to be purchased and stored.
In addition, these ammonium salt-containing solutions produce waste water flows which cannot be discharged into the sewer system without further ado. Here as well, a purification is to be carried out and a solution for the disposal of the produced saline solution found.
Another process according to the state of the art for the removal of ammonia from waste gas flows is flare combustion. To ensure a minimum calorific value, it is necessary, however, to provide significant amounts of auxiliary gas depending on the concentration of the ammonia and other combustible pollutants, which will cause an increase of the carbon dioxide emissions of the plant. The ammonia combustion implemented in the flare also leads to the formation of nitrogen oxides which constitute, as already mentioned above, environmental pollutants themselves, so that the formation of these noxious substances should be avoided. Furthermore there are indications that the authorities in Europe consider open flares to be problematic for continuous operation as it is not possible to measure the emissions.
Another process for the purification of ammonia-containing waste gas flows is the selective oxidation described in DE 695 30 024 T2 and EP 0 514 729 B1, for example, which is carried out by means of specific catalysts. In the case of higher concentrations, however, adequate cooling is to be provided for the amounts of dissipated heat, which will increase the demand for equipment. Consequently this process is mainly suited for flows which are only polluted with minor amounts of ammonia.
Patent DE 4314896 A1 describes the depletion of ammonia by purposeful addition of nitrogen oxides. The nitrogen oxides required are produced by a separate source (e.g. diesel engine) or by partial oxidation of ammonia. The reaction of the ammonia and the nitrogen oxides in the presence of a catalyst essentially takes place according to the below reaction equations:4NO+4NH3+O2→4N2+6H2O  (1)6NO2+8NH3→7N2+12H2O  (2)The waste gas temperatures specified in this patent require that catalysts be used to increase the reaction velocity. Apart from the high investment costs, catalysts involve the risk of clogging from waste gases with solid pollutants (e.g. ash) or even being deactivated. Furthermore, an artificial nitrogen oxide source is required for the purification of the ammonia-containing waste gas.
A similar purification process is mentioned in EP1350552B1. In a first step, the ammonia is combusted in a thermal post-combustion plant, which will inevitably produce nitrogen oxides. The aim of the invention is to not combust the ammonia completely and to thus use the non-combusted ammonia portion in a further step for the selective catalytic reduction of the nitrogen oxides. This process makes it necessary to install a sophisticated control system.
Another alternative for the depletion of ammonia in waste gases is the adsorption onto, for example, activated carbon or zeolites as disclosed by U.S. Pat. No. 6,261,345 B1. A disadvantage of this process is unsteady operation which will increase the demand for equipment.
The physical absorption of ammonia in the presence of a solvent such as water or glycols is also a process according to the state of the art as described in U.S. Pat. No. 5,230,877. The reachable purities of the waste gas, however, largely depend on the working conditions of the scrubbing unit such as temperature and pressure. Of advantage are high pressure and low temperature, which will, however, incur additional investment and operating cost.
Nitrogen oxides may be formed during the combustion of fossil fuels or are obtained during the production of nitric acid. Nitrogen oxides are detrimental to health and the environment and contribute to such phenomena as acid rain, smog and the formation of ozone, statutory maximum emission values for this substance being required for this reason. The limit value for boilers operated by natural gas recommended by the Technical Instructions on Air Quality Control, First General Administrative Regulation pertaining to the Federal Air Pollution Control Act, as of 2002, currently is 150 mg/Nm3.
The most obvious procedure to achieve a depletion of nitrogen oxides is to modify the causative sources in an adequate way. Therefore special nitrogen oxide-lean burners can, for example, be used in heating boilers and/or the combustion including air pre-heating and waste gas return can be adjusted as required. Problematic in this connection is that the boundary conditions of the process may be opposed to such adjustment and that, from the financial point of view, the measures may additionally get quite expensive.
Widely used for the removal of nitrogen oxides from waste gases are, in particular, selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR), in which the ammonia and nitrogen oxides essentially react in the gas phase (SNCR) or on the catalyst surface (SCR) in compliance with the before-mentioned reaction equations (1) and (2).
The selective non-catalytic reduction (SNCR) takes place at a temperature between 850° C. and 1100° C. Here, an aqueous ammonia or urea solution usually serves as reducing agent. A temperature above 900° C. is required to reach the necessary reaction velocity. The process involves the problem that at temperatures above 1000° C. a higher amount of nitrogen oxides is formed from ammonia according to the following reaction equation:4NH3+5O2→4NO+6H2O  (3)
In contrast to this, the selective catalytic reduction takes place at temperatures between 150° C. and 550° C. and the risk of an increased nitrogen oxide formation is to be regarded as rather low. This process, however, requires the use of a catalyst. Such catalysts mainly consist of heavy metal oxides, zeolites, activated char and iron oxide-chromium oxides.
SNCR and SCR may also be combined with each other to utilise the advantages of both processes (low nitrogen oxide emissions and low catalyst demand) (e.g. pursuant to DE 690 06 367 T2). This makes it necessary, however, that the waste gas is of the respective temperatures required by both processes.
Main fields of application of SNCR and SCR are, for example, the nitrogen oxide depletion of waste gases from power stations and waste incineration plants and the purification of waste air from nitric acid production plants. The SCR process has recently been used for the depletion of nitrogen oxides in automobile emissions. The use of SNCR and SCR in ammonia/urea production plants does so far not constitute the state of the art.
The state of the art only discloses to treat and purify such waste gas flows that contain ammonia and/or nitrogen oxides separately of each other according to the before-mentioned methods. Such methods are very equipment-intensive and in addition extremely cost-intensive.